Anti-foulant in acrylonitrile manufacture

ABSTRACT

Lignosulfonate metal salts are added to the water used to absorb acrylonitrile to minimize deposits which otherwise foul the heat transfer surfaces of the heat exchangers used to cool the water before recycling it to the system.

United States Patent Halvorson et al.

[ 51 Sept. 12, 1972 ANTI-FOULANT IN ACRYLONITRILE MANUFACTURE Inventors:David 0. l-lalvorson, Pitman, N.J.;

Sterling N. Vines, Memphis, Tenn.

Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

Filed: Nov. 12, 1970 Appl. No.: 88,991

US. Cl ..260/465.9, 260/465.3 Int. Cl ..C07c 121/32 Field of Search..260/465.9, 465.3

References Cited 3,535,849 10/1970 l-lausweiler et al..260/465 .9 X

Primary Examiner-Joseph P. Brust Attorney-Samuel S. Blight [57] ABSTRACTLignosulfonate metal salts are added to the water used to absorbacrylonitrile to minimize deposits which otherwise foul the heattransfer surfaces of the heat exchangers used to cool the water beforerecycling it to the system.

11 Claims, No Drawings ANTI-FOULANT IN ACRYLONITRILE MANUFACTUREBACKGROUND OF THE INVENTION This invention is directed toward a processfor the preparation of acrylonitrile and, more particularly, to aprocess wherein certain lignosulfonates are added to the water which wasused to absorb the acrylonitrile before the water is cooled and recycledto the absorption step.

When acrylonitrile is made by the ammoxidation of propylene in agas-phase catalytic reaction the product gases are colled and unreactedammonia in the product gases is neutralized. These cooled gases are thenpassed to an absorber in which the acrylonitrile is recovered byabsorption in a suitable solvent, e.g. chilled water. The absorbedacrylonitrile is recovered by distillation and a portion of the bottomsfrom the distillation is cooled and recycled to the absorption step.

This recycle contains both inorganic and organic compounds in the formof monomers, oligomers, prepolymers, and polymers in variouscombinations. These compounds foul the heat exchange surfaces of theheat exchangers used to cool the solvent prior to recycle. In order tominimize this fouling when water is employed as the solvent, a portionof the recycle water must be purged and replenished with fresh water.The purge retards formation of solids on the heat exchanger surfaces byremoving some of the foulants, but purging is costly because the purgeis toxic and must incinerated. Fouled heat exchanger surfaces reduce thecoefficient of heat transfer thereby increasing the amount of coolantwhich must be employed to realize the same amount of cooling obtained onclean surfaces. Eventually, the heat exchanger must be manually cleaned.Reduction of the water purge and/or fouling of the heat exchangersoffers improved economics.

SUMMARY OF THE INVENTION It has now been discovered that lignosulfonatesalts of alkali and alkaline earth metals which when added to thesolvent for the acrylonitrile will substantially reduce, if noteliminate, deposits of materials which cause fouling of the heattransfer surfaces when the solvent is cooled.

DETAILED DESCRIPTION OF THE INVENTION In one method for the manufactureof acrylonitrile gaseous reactants from the gas phase ammoxidation ofabout 400-5l0C. and any ammonia is neutralized by passing the gascountercurrent to an aqueous stream of acid such as sulfuric acid, toneutralize and recover any ammonia present such as disclosed in US.Pats. Nos. 3,404,947 and 3,408,157. The resultant gases which containmajor amounts of nitrogen and acrylonitrile and minor amounts ofhydrogen cyanide, acetonitrile, carbon dioxide, carbon monoxide,propylene, ammonia, water, oxygen, acrolein and certain acid aldehydesand nitriles are contacted with water at at temperature of l40C. to forma solution containing less than about 10 percent by weightacrylonitrile. The acrylonitrile (along with some water and hydrogencyanide) is separated from any acetonitrile present by distillation andrecovered overhead. Volatiles are separated from the resultant aqueousstream in a stripper. The bottoms from the stripper containapproximately 1 percent organic material giving the bottoms the typicalanalysis:

160 20 120 500 500 5400 300 as well as water-soluble polymers such aspolyacrylic acid and its salts.

It is convenient to cool a substantial portion of this bottoms stream toa temperature in the range 1-40C. before it is recycled and contactedwith any substantial amount of acrylonitrile. As the process is operatedmaterials form which foul the heat exchangers thereby decreasing heattransfer efficiency. In order to remove the foulants and minimizefouling, a portion of the bottoms stream is usually continuously purgedfrom the system and is replace with water which is free from foulingimpurities. Although the concentration of impurities reaches equilibriumduring the operation of the process, the buildup of foulants continuesand eventually the cooling system must be manually cleaned to restoreefficiency.

The antifouling agents which are added to the bottoms are referred togenerally as lignosulfonates, are water soluble polyelectrolytes, aredispersants and are in the form of alkali metal salts and alkaline earthmetal salts and mixtures thereof. The alkali metal lignosulfonates arepreferred. Although it is not known with certainty, the structure of thesodium salt is believed to ppm hydrogen cyanide ppm acrylonitrile ppmacetic acid ppm fumaronitrile ppm nicotinonitrile ppm succinonitrile ppmmaleonitrile propylene are cooled from an initial temperature of 50 beof the following type:

forms. For example, a satisfactory form is a nearly black aqueoussolution of the natural organic polyelectrolyte which is sold under thetrade name Betz 402 (Sp. Gr.l.145, Viscosity at 60F. 21 cps), the solidsportions of which constitutes approximately 30 percent by weight of theformulation. These solids exhibit a spectrum which matches that of knownsodium lignosulfonate and analyze 47.2 percent carbon, 4.7 percenthydrogen, 5.7 percent sulfur in addition to sodium and no nitrogen. Thevolatile portions of this commercial lignosulfonate are water and minoramounts of methanol, acetone, dimethyl sulfide and omethoxyphenol. Theconcentration of the lignosulfonates employed will be dictated by thedegree of dispersion desired which in turn is directly related to theamount of purge taken and is usually in the range 0.0015-1 .0 percent byweight based upon the weight of the recycle water and preferably ispresent at a concentration on the range 0.l-0.2 on the same basis.Generally, the amount of sulfonate required will increase as the amountof purge is decreased. The lignosulfonates may be introduced at anylocation in the recycle water circuit, e.g. it may be desirable to addthe sulfonates to the stream from the absorber and before the strippingcolumn thereby permitting the removal of any undesirable volatilematerial which may be present in commercially available sulfonates.

The recycle water should be maintained at a pH of 3-7 and preferably of5-6 through the stripping and cooling operations in order to realize themaximum effect of the polyelectrolyte and to minimize product loss whichinevitably occurs at higher pH.

The following examples are presented to illustrate but not to restrictthe present invention. Parts and percentages are by weight unlessotherwise noted.

EXAMPLE 1 Approximately 900 grams of stripper bottoms having the typicalanalysis set forth hereinabove along with 0.9 gram of an aqueoussolution of a sodium lignosulfonate which was marketed under the tradename Betz 402 (as described hereinabove) were placed in a resin potwhich was equipped with a stirrer and heater. The concentration ofsodium lignosulfonate was 0.03 percent by weight based upon the weightof the bottoms. A U- tube of stainless steel having a smooth outersurface was arranged to extend into the pot approximately 1 inch fromthe sides thereof and secured such that the length of the tube which wasimmersed in the liquid was constant. The contents of the pot were heatedin the range 5060C. for 24 hours during which time water at atemperature of 14C. was passed through the tube at a rate such that exittemperature was maintained between 16 and 17C. After the aforementionedperiod. the U-tube had only a light brown coating weighing 0.0016 gram.When the foregoing experiment was repeated in the absence of thelignosulfonate, heavy fouling of the tube occurred with the weight offoulant ranging between about 0.0051 and 0.0104 gram.

EXAMPLE 2 Example 1 was repeated with the exception that 28 grams ofacrylonitrile were added to the resin pot. No adverse effect on theaction of the lignosulfonate was observed.

EXAMPLE 3 Example 1 was repeated except that the contents of the potwere boiled under reflux for one hour before the cooling coil wasimmersed. No adverse effect on the action of the lignosulfonate wasobserved.

EXAMPLE 4 20 grams of this concentrate was treated as in Example 1 butwithout the sulfonate. The inlet water temperature was 20C. and flow ofexit water was adjusted to maintain its temperature in the range 22-25C.The U-tube was heavily fouled with 0.0802 gram of a brown-black solid.The example was then repeated with the concentrate containing 0.9 gramof the aqueous solution of sodium lignosulfonate of Example 1. Thecooling surface was only fouled with 0.0417 gram of a lightbrown-blacksolid.

EXAMPLE 5 A sample of stripper bottoms having the typical analysis setforth above was concentrated to percent volume by distillation and 900grams of this concentrate were treated as in Example 1 but without thelignosulfonate. The stainless steel U-tube was fouled with 0.0130 gramof a brown solid. When the foregoing was repeated, but with the additionof 1.8 grams of the aqueous solution of the lignosulfonate of Example 1,

only a trace of fouling occurred which amounted to 0.0027 gram.

Substantially similar results were obtained when a mixed calcium,magnesium and sodium lignosulfonate sold under the trade name Marasperse021, or Marasperse N-22" (a desulfonated lignosulfonate) weresubstituted for the lignosulfonate of Example 1. Circulation of thestripper bottoms through external apparatus did not adversely affect theperformance of the lignosulfonate.

The lignosulfonates are equally effective when lean water is cooled to atemperature 1-l0C.

We claim:

1. In a process for the preparation of acrylonitrile which comprisesabsorbing gaseous acrylonitrile from a gas phase ammoxidation reactioninto a solvent, recovering said acrylonitrile by distillation from thesolvent and thereafter cooling and recycling a portion of the solvent,the improvement which comprises cooling a portion of the recycledsolvent in the presence of 0.0015-1 percent by weight based upon theweight of said portion of an alkali or alkaline earth metallignosulfonate.

2. The process of claim 1 wherein the solvent is water.

3. The process of claim 2 wherein the water is cooled to a temperaturein the range l40C.

4. Theprocess of claim 3 wherein said water is maintained at a'pltl inthe range 3-7.

5. The process of claim 2 wherein said lignosulfonate is substantiallysodium lignosulfonate.

6. The process of claim 5 wherein said lignosulfonate is present at aconcentration in the range 0.015 to 0.20 percent by weight based uponthe weight of said portion.

7. In a process for the preparation of acrylonitrile which comprisesquenching the gases from the gas phase ammoxidation of propylene,ammonia and molecular oxygen, absorbing the acrylonitrile from thequenched gases in water at a temperature in the range l40C., recoveringacrylonitrile by distillation from the water and thereafter recycling aportion of the water, the improvement which comprises cooling a portionof the recycle water from which the acrylonitrile has been recovered inthe presence of 0.00l5-1 percent by weight based upon the weight of theportion of said water of an alkali or alkaline earth metallignosulfonate and recycling the portion thus cooled to the absorptionstep.

8. The process of claim 7 wherein the water is maintained at a pH in therange 3-7 during cooling.

9. The process of claim 7 wherein the lignosulfonate is sodiumlignosulfonate.

10. The process of claim 8 wherein the metal of the lignosulfonate is amixture of calcium, magnesium and sodium.

11. The process of claim 8 wherein the water is cooled to a temperaturein the range l-40C.

2. The process of claim 1 wherein the solvent is water.
 3. The processof claim 2 wherein the water is cooled to a temperature in the range 1*-40*C.
 4. The process of claim 3 wherein said water is maintained at apH in tHe range 3-7.
 5. The process of claim 2 wherein saidlignosulfonate is substantially sodium lignosulfonate.
 6. The process ofclaim 5 wherein said lignosulfonate is present at a concentration in therange 0.015 to 0.20 percent by weight based upon the weight of saidportion.
 7. In a process for the preparation of acrylonitrile whichcomprises quenching the gases from the gas phase ammoxidation ofpropylene, ammonia and molecular oxygen, absorbing the acrylonitrilefrom the quenched gases in water at a temperature in the range 1*-40*C., recovering acrylonitrile by distillation from the water andthereafter recycling a portion of the water, the improvement whichcomprises cooling a portion of the recycle water from which theacrylonitrile has been recovered in the presence of 0.0015-1 percent byweight based upon the weight of the portion of said water of an alkalior alkaline earth metal lignosulfonate and recycling the portion thuscooled to the absorption step.
 8. The process of claim 7 wherein thewater is maintained at a pH in the range 3-7 during cooling.
 9. Theprocess of claim 7 wherein the lignosulfonate is sodium lignosulfonate.10. The process of claim 8 wherein the metal of the lignosulfonate is amixture of calcium, magnesium and sodium.
 11. The process of claim 8wherein the water is cooled to a temperature in the range 1* -40*C.